But others are concerned about the high cost of such research and question whether it should be conducted at a profit-making hospital, in this case one owned by Humana Inc., a Louisville-based corporation.

* Can the nation afford all of the costly medical innovations now emerging from laboratories?

* When businesses get involved in medical research, will concern about the bottom line harm the public good?

The past two decades have seen a virtual explosion in new medical tools and know-how. At the same time, medical-care costs have ratcheted upward, with Americans spending some 10.5 percent of the gross national product on medical care last year - twice the amount in 1960.

Although not the only cause of the nation's rising health tab, new medical developments have played a part. A recent study by the congressional Office of Technology Assessment found that increased use of medical technology, such as new drugs, diagnostic devices, and surgical procedures, accounts for nearly one-third of the increase in medicare costs over the past five years.

''It (the artificial heart) is the prototype of a whole series of technologies that are coming on stream and are going to confront us with grave decisions about whether we want to help (only) a limited number of people,'' argues Dr. William B. Schwartz, a professor of medicine at Tufts University School of Medicine.

Some health planners and others in the medical community worry that without some check on costs, the nation will be forced to ration medical services. The rapidly developing field of organ transplants, considered one of modern medicine's major achievements, is already forcing tough public-policy decisions about who should pay and who should benefit from them. The artificial heart is the latest lightning rod in the debate.

''We're reaching the point with scientific advancement and technological development where our capacities to engage in life-extending work are beginning to outstrip our ability to pay,'' says Dr. Harvey V. Fineberg, dean of the Harvard School of Public Health. ''The idea of there being limits on resources is not a futuristic notion. It is with us today.''

The debate is sure to continue, given the experience of the latest recipient of a mechanical heart, William Schroeder. Just over a week after his operation, Mr. Schroeder was making enough progress for doctors to test a briefcase-sized power pack to drive the pump. Such a device, instead of a refrigerator-sized console now in use, is considered an essential first step if recipients are ever to enjoy greater mobility than that afforded by the current console.

Eventually, some estimate that as many as 50,000 people a year in the US may be able to benefit from such devices. Enthusiasts concede that the procedure is costly now - each implant has been estimated to cost from $100,000 to $250,000. But they point out that those figures may drop. Even at $100,000, the price would be on par with human heart transplants. Many argue that virtually no price is too high to keep a person alive.

Others contend that economics should not be an issue at all: The research is still in too early a stage to worry about costs. They argue that all new medical work can't be suspended just because it might be costly in the short term.

''The widespread application of this technology is not for this decade,'' says Dr. Pierre Galletti, vice-president for biology and medicine at Brown University. He says the money going into the artificial-heart field is ''peanuts'' compared to the development of, say, a new drug. ''It's too early for that kind of debate.''

On the other hand, critics caution that there is a technological imperative involved with some of these new techniques. If developed now, they are hard to stop later on, even if there might be compelling reasons to do so. Moreover, some observers don't see prices dropping very much, even if the procedure becomes routine. A good deal of the costs involved in such implants pay for intensive care and surgeons' fees, not the mechanical heart and other devices themselves.

Arthur Caplan of the Hastings Center, a New York State research institute dealing with ethical problems in medical care, argues that even if the price were to drop below $100,000 per implant, doing 50,000 to 60,000 of them a year would mean a multibillion-dollar program to help a relatively small fraction of the nation's people medically diagnosed as having heart disease. Critics, too, say they believe that such money could be better spent in other research areas, including programs aimed at ''preventing'' heart disease. Dr. Fineberg and others argue that more lives might be saved in the long run by spending more on educating people to stop smoking or on gaining a better understanding of the underlying causes of the disease.

Instead, worrisome to him and others in the medical community is the idea of a for-profit corporation like Humana, which has promised to finance up to 100 implants, getting involved in such experimental work. Questions about health-care companies that must keep one eye on patients' well-being and the other on the bottom line aren't new. But the Humana case shifts the focus to medical research.

The concern is that an emphasis on profit might dilute some of the purer aspects of research. Critics argue that such a dangerous and experimental frontier would be better carried out and evaluated at a university or other nonprofit center, where decisions on where to spend basic research dollars or whether to forge ahead with a technology would theoretically be free of profit or publicity motives. Some also worry that a flush corporation like Humana, with almost $3 billion a year in revenues, might eventually gain a monopoly on the technology.

Humana officials counter that their work will benefit stockholders and society. They note that private investment in such areas can help offset government funding cuts at major academic centers. They also point out that Dr. William DeVries, the head of the artificial heart team who came to the Louisville hopsital from the University of Utah, has been freed from fund raising and devotes more time now to doing scientific work. To avoid the appearance of a conflict of interest, both Dr. DeVries and Humana have also sold their interests in the firm that manufactures the artificial heart.